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η Sagittarii
Location of η Sagittarii (circled)
Observation data
Epoch J2000       Equinox J2000
Constellation Sagittarius
Right ascension 18h 17m 37.63505s [1]
Declination −36° 45′ 42.0667″ [1]
Apparent magnitude (V) +3.11 [2]/+7.8
Characteristics
Spectral type M2 III [3] + F7 V [4]
U−B color index +1.71 [2]
B−V color index +1.56 [2]
Variable type Lb [5]
Astrometry
Radial velocity (Rv)+0.5 [6] km/s
Proper motion (μ) RA: −129.56 [1]  mas/ yr
Dec.: −166.26 [1]  mas/ yr
Parallax (π)22.35 ± 0.24  mas [1]
Distance146 ± 2  ly
(44.7 ± 0.5  pc)
Absolute magnitude (MV)−4.90 [7]
Details
η Sgr A
Radius57 [8]  R☉
Other designations
η Sagittarii, η Sgr, β Telescopii, β Tel, CCDM J18176-3646AB, CD−36°12423, CPD-36°8128, FK5 683, GC 24944, HD 167618, HIP 89642, HR 6832, IDS 18109-3647 AB, PPM 297508, SAO 209957, WDS J18176-3646AB. [9]
Database references
SIMBAD data

Eta Sagittarii (Eta Sgr, η Sagittarii, η Sgr) is a binary star system in the southern zodiac constellation of Sagittarius. Based upon parallax measurements, it is located at a distance of 146 light-years (45 parsecs) from Earth. [1] In India, where part of the constellation of Sagittarius represents an Elephant, this star forms the creature's tail. [10]

The primary component, η Sagittarii A, is a red giant star with a stellar classification of M2 III. [3] It is an evolved star that is currently at a stage called the asymptotic giant branch, having exhausted both the hydrogen and the helium at its core. [11] This star is classified as an oxygen-rich irregular variable, [7] as it undergoes small magnitude fluctuations between +3.08 and 3.12. [4] The measured angular diameter of this star is 11.9 ± 2.1 mas. [12] At the estimated distance of Eta Sagittarii, [1] this yields a physical size of about 57 times the radius of the Sun. [8]

The companion, η Sagittarii B, was first noted by American astronomer S. W. Burnham in 1879. The two stars share a common proper motion and hence are probably gravitationally bound to each other. [13] The secondary is likely an F-type main sequence star with an apparent magnitude of +7.77. It located at an angular separation of 3.6  arcseconds from the primary, along a position angle of 108°. [14] This star is at a projected distance of 165 Astronomical Units from the red giant primary and the pair take a minimum of 1,270 years to complete an orbit. [4]

Within the context of the Milky Way galaxy, this system is a member of the faint old disk group. [7] Because of proper motion, this star will move into constellation Corona Australis around 6300 CE. [15] Eta Sagittarii has two optical companions that are not physically associated with the system. The first is a 10th magnitude star at an angular separation of 93 arcseconds with a position angle of 303°. There is a fainter, 13th magnitude star at an angular separation of 33 arcseconds along a position angle of 276°. [13]

Name and etymology

  • This star, together with γ Sgr, δ Sgr and ε Sgr were Al Naʽām al Wārid (النعم الوارد), the Going Ostriches. [16] According to the catalogue of stars in the Technical Memorandum 33-507 - A Reduced Star Catalog Containing 537 Named Stars, Al Naʽām al Wārid or Namalwarid was the title for this star [17]
  • In the catalogue of stars in the Calendarium of Al Achsasi al Mouakket, this star was designated Rabah al Waridah or Rabi al Waridah, meaning fourth of Warida. [18]
  • In Chinese, 箕 (Jī), meaning Winnowing Basket, refers to an asterism consisting of η Sagittarii, γ Sagittarii, δ Sagittarii and ε Sagittarii. Consequently, the Chinese name for η Sagittarii itself is 箕宿四 (Jī Sù sì, English: the Fourth Star of Winnowing Basket.) [19]

References

  1. ^ a b c d e f g van Leeuwen, F. (November 2007), "Validation of the new Hipparcos reduction", Astronomy and Astrophysics, 474 (2): 653–664, arXiv: 0708.1752, Bibcode: 2007A&A...474..653V, doi: 10.1051/0004-6361:20078357, S2CID  18759600.
  2. ^ a b c Nicolet, B. (1978), "Photoelectric photometric Catalogue of homogeneous measurements in the UBV System", Astronomy and Astrophysics Supplement Series, 34: 1–49, Bibcode: 1978A&AS...34....1N.
  3. ^ a b Houk, Nancy (1979), "Michigan catalogue of two-dimensional spectral types for the HD stars", Michigan Catalogue of Two-dimensional Spectral Types for the HD Stars. Volume_3. Declinations -40_ƒ0 to -26_ƒ0, 3, Ann Arbor, Michigan: Dept. of Astronomy, University of Michigan, Bibcode: 1982mcts.book.....H.
  4. ^ a b c Kaler, James B., "Eta Sagittarii", Stars, University of Illinois, retrieved 2012-02-10.
  5. ^ Samus, N. N.; Durlevich, O. V.; et al. (2009). "VizieR Online Data Catalog: General Catalogue of Variable Stars (Samus+ 2007-2013)". VizieR On-line Data Catalog: B/GCVS. Originally Published in: 2009yCat....102025S. 1. Bibcode: 2009yCat....102025S.
  6. ^ Wilson, R. E. (1953), "General Catalogue of Stellar Radial Velocities", Washington, Carnegie Institute of Washington D.C., Bibcode: 1953GCRV..C......0W.
  7. ^ a b c Mennessier, M. O.; et al. (August 2001), "Long period variable stars: galactic populations and infrared luminosity calibrations", Astronomy and Astrophysics, 374 (3): 968–979, arXiv: astro-ph/0105552, Bibcode: 2001A&A...374..968M, doi: 10.1051/0004-6361:20010764, S2CID  15721872.
  8. ^ a b Lang, Kenneth R. (2006), Astrophysical formulae, Astronomy and astrophysics library, vol. 1 (3 ed.), Birkhäuser, ISBN  3-540-29692-1. The radius (R*) is given by:
  9. ^ "HD 167618 -- Variable Star", SIMBAD Astronomical Database, retrieved 2007-03-29.
  10. ^ O'Meara, Stephen James (2011), Deep-Sky Companions: The Secret Deep, Cambridge University Press, p. 341, ISBN  978-0-521-19876-9.
  11. ^ Eggen, Olin J. (July 1992), "Asymptotic giant branch stars near the sun", Astronomical Journal, 104 (1): 275–313, Bibcode: 1992AJ....104..275E, doi: 10.1086/116239.
  12. ^ Richichi, A.; Percheron, I. (May 2005), "First results from the ESO VLTI calibrators program", Astronomy and Astrophysics, 434 (3): 1201–1209, arXiv: astro-ph/0501532, Bibcode: 2005A&A...434.1201R, doi: 10.1051/0004-6361:20042257, S2CID  2847613
  13. ^ a b Burnham, Robert (1978), Burnham's Celestial Handbook: an observer's guide to the universe beyond the solar system, Dover books explaining science, vol. 3 (2nd ed.), Courier Dover Publications, ISBN  0-486-23673-0.
  14. ^ Jasinta, D. M. D.; Soegiartini, E. (October 1994), "Photographic observations of visual double stars", Astronomy and Astrophysics Supplement, 107: 235–241, Bibcode: 1994A&AS..107..235J.
  15. ^ Moore, Patrick; Rees, Robin (2011), Patrick Moore's Data Book of Astronomy, Cambridge: Cambridge University Press, p. 296, ISBN  9781139495226.
  16. ^ Allen, R. H. (1963). Star Names: Their Lore and Meaning (Reprint ed.). New York: Dover Publications Inc. p.  355. ISBN  0-486-21079-0. Retrieved 2012-09-04.
  17. ^ Jack W. Rhoads - Technical Memorandum 33-507-A Reduced Star Catalog Containing 537 Named Stars, Jet Propulsion Laboratory, California Institute of Technology; November 15, 1971
  18. ^ Knobel, E. B. (June 1895). "Al Achsasi Al Mouakket, on a catalogue of stars in the Calendarium of Mohammad Al Achsasi Al Mouakket". Monthly Notices of the Royal Astronomical Society. 55 (8): 430. Bibcode: 1895MNRAS..55..429K. doi: 10.1093/mnras/55.8.429.
  19. ^ (in Chinese) AEEA (Activities of Exhibition and Education in Astronomy) 天文教育資訊網 2006 年 5 月 11 日 Archived 2011-05-22 at the Wayback Machine
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